I don't know if this thread is still active, I've lived in Hanoi for over 1.5 years. The north has some amazing countryside for riding, great karst topography and mountains. As far as getting a bike, you can get the 175cc or larger bikes, but to do it fully legal is a big hassle. I'd suggest getting a 125 or 150cc bike. Most guys I know have a 125, I've got a 150 - and every little bit helps. Some fellows get the 170cc Rebel, but that may be hard to come by. Also, you can get large bikes on The New Hanoian, but you may be riding "less than legal". I don't know if you are familiar with that site, but I've found TNH to be very helpful.
Again welcome to Vietnam, and post some pictures from your trips around the north. I'd recommend a week up to Ha Giang sometime.